Metrology Apparatus and a Method of Determining a Characteristic of Interest

ABSTRACT

A computer program product causes a processor to execute a process of causing an optical system to illuminate at least one structure on a substrate that comprises first repetitive features at a first pitch in a first layer and second repetitive features at a second pitch in a second layer, the first repetitive features at least partially overlapping with the second repetitive features. The first pitch is different from the second pitch. The processor causes the optical system to receive radiation scattered by the at least one structure and transmit a portion of the received scattered radiation to a sensor arranged in an image plane of the optical system or in a plane conjugate with the image plane for detecting the received scattered radiation and configured to detect a characteristic of radiation impinging on the sensor. The processor then determines a characteristic of interest of the structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/028,287, filed on Sep. 22, 2020, which is a continuation of U.S.patent application Ser. No. 16/826,479, filed on Mar. 23, 2020, now U.S.Pat. No. 11,327,410, issued May 10, 2022, which is a divisional of U.S.patent application Ser. No. 16/181,455, filed on Nov. 6, 2018, now U.S.Pat. No. 10,809,632, issued Oct. 20, 2020, which claims benefit ofEuropean Application No. 17207587.1; filed Dec. 15, 2017, EuropeanApplication No. 17204158.4, filed Nov. 28, 2017 and European ApplicationNo. 17200265.1, filed on Nov. 7, 2017, which are all incorporated byreference herein in their entirety.

FIELD

The present invention relates to a metrology apparatus for determining acharacteristic of interest of a structure on a substrate. The presentinvention also relates to a method of determining a characteristic ofinterest.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ, is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k₁.

In lithographic processes, it is desirable to make frequent measurementsof the structures created, e.g., for process control and verification.Various tools for making such measurements are known, including scanningelectron microscopes or various forms of metrology apparatuses, such asscatterometers. A metrology apparatus may be operable to determine anoverlay value between two overlapping layers on a substrate. If theoverlay value deviates from an expected value, the metrology apparatuscan report the deviation from the expected value as an overlay error.

SUMMARY

It is an object to provide an effective and efficient solution for aninspection or metrology apparatus that is better than the knownmetrology or inspection apparatuses.

According to an aspect of the invention a metrology apparatus isprovided as defined in the claims. According to a further aspect of theinvention a method of determining a characteristic of interest relatingto at least one structure on a substrate is provided as defined in theclaims. The claims are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic representation of an embodiment of ametrology apparatus;

FIG. 5 depicts a schematic representation of a second embodiment of ametrology apparatus;

FIG. 6 depicts a number of schematic representations of structures thatmay be manufactured on a substrate and that may be inspected by theembodiments of the metrology apparatus;

FIG. 7 depicts an embodiment of images that may be obtained by a sensorof a metrology apparatus;

FIG. 8 depicts a schematic representation of a third embodiment of ametrology apparatus;

FIG. 9 depicts a schematic representation of a fourth embodiment of ametrology apparatus; and

FIG. 10 depicts schematic representations of structures to bemanufactured on the substrate to determine overlay values;

FIG. 11 depicts schematic representations of sub-segmentations ofembodiments of targets;

FIG. 12 depicts schematic representations of illumination pupils anddetection pupils for the metrology apparatus;

FIGS. 13(a) and 13(b) schematically depicts in FIGS. 13(a) and 13(b)examples of flexible and/or controllable illumination and detectionpupil arrangements,

FIGS. 14(a) and 14(b) schematically depicts in FIGS. 14(a) and 14(b)further examples of flexible and/or controllable illumination anddetection pupil arrangements,

FIG. 15 schematically depicts a further embodiment of a target.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV/soft-X-ray radiation (extreme ultra-violet radiation, e.g. having awavelength in the range of about 1-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) T constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection systems,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W— which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support T, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1 ) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3 . One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurement are typically calledmetrology too

Is MT. Different types of metrology tools MT for making suchmeasurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT. Scatterometers areversatile instruments which allow measurements of the parameters of alithographic process by having a sensor in the pupil or a conjugateplane with the pupil of the objective of the scatterometer, measurementsusually referred to as pupil based measurements, or by having the sensorin the image plane or a plane conjugate with the image plane, in whichcase the measurements are usually referred to as image or field basedmeasurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers may measure gratings using light from softx-ray and visible to near-IR wavelength range.

In a first embodiment, the scatterometer MT is an angular resolvedscatterometer. In such a scatterometer reconstruction methods may beapplied to the measured signal to reconstruct or calculate properties ofthe grating. Such reconstruction may, for example, result fromsimulating interaction of scattered radiation with a mathematical modelof the target structure and comparing the simulation results with thoseof a measurement. Parameters of the mathematical model are adjusteduntil the simulated interaction produces a diffraction pattern similarto that observed from the real target.

In a second embodiment, the scatterometer MT is a spectroscopicscatterometer MT. In such spectroscopic scatterometer MT, the radiationemitted by a radiation source is directed onto the target and thereflected or scattered radiation from the target is directed to aspectrometer detector, which measures a spectrum (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile of the target givingrise to the detected spectrum may be reconstructed, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra.

In a third embodiment, the scatterometer MT is a ellipsometricscatterometer. The ellipsometric scatterometer allows for determiningparameters of a lithographic process by measuring scattered radiationfor each polarization state. Such metrology apparatus emits polarizedlight (such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

In one embodiment of the scatterometer MT, the scatterometer MT isadapted to measure the overlay of two misaligned gratings or periodicstructures by measuring asymmetry in the reflected spectrum and/or thedetection configuration, the asymmetry being related to the extent ofthe overlay. The two (typically overlapping) grating structures may beapplied in two different layers (not necessarily consecutive layers),and may be formed substantially at the same position on the wafer. Thescatterometer may have a symmetrical detection configuration asdescribed e.g. in co-owned patent application EP1,628,164A, such thatany asymmetry is clearly distinguishable. This provides astraightforward way to measure misalignment in gratings. Furtherexamples for measuring overlay error between the two layers containingperiodic structures as target and measured through asymmetry of theperiodic structures may be found in PCT patent application publicationno. WO2011/012624A1 or US patent application US20160161863A1,incorporated herein by reference in their entirety.

In yet another embodiment of the scatterometer MT, the scatterometer MTis adapted to block the zeroth order diffracted radiation, and to formimages based on first or higher diffracted orders, wherein the imagesare not resolving the structures present on the wafer. Such apparatus isa dark field metrology apparatus and examples of diffraction basedmetrology in a dark field setup are described in international patentapplications WO2009/078708A1 and WO2009/106279A1, which documents areincorporated herein by reference in their entirety. Also documentUS2006/0098199A1 is incorporated herein by reference in its entirety.

Other parameters of interest may be focus and dose. Focus and dose maybe determined simultaneously by scatterometry (or alternatively byscanning electron microscopy) as described in US patent applicationUS2011-0249244, incorporated herein by reference in its entirety. Asingle structure may be used which has a unique combination of criticaldimension and sidewall angle measurements for each point in a focusenergy matrix (FEM— also referred to as Focus Exposure Matrix). If theseunique combinations of critical dimension and sidewall angle areavailable, the focus and dose values may be uniquely determined fromthese measurements.

A metrology target may be an ensemble of composite gratings, formed by alithographic process, mostly in resist, but also after etch process forexample. Typically the pitch and line-width of the structures in thegratings strongly depend on the measurement wavelength and/or themeasurement optics (in particular the NA of the optics) to be able tocapture diffraction orders coming from the metrology targets. Asindicated earlier, the diffracted signal may be used to determine shiftsbetween two layers (also referred to as ‘overlay’) or may be used toreconstruct at least part of the original grating as produced by thelithographic process. This reconstruction may be used to provideguidance of the quality of the lithographic process and may be used tocontrol at least part of the lithographic process. Targets may havesmaller sub-segmentation which are configured to mimic dimensions of thefunctional part of the design layout in a target. Due to thissub-segmentation, the targets will behave more similar to the functionalpart of the design layout such that the overall process parametermeasurements resemble the functional part of the design layout better.The targets may be measured in an underfilled mode or in an overfilledmode. In the underfilled mode, the measurement beam generates a spotthat is smaller than the overall target. In the overfilled mode, themeasurement beam generates a spot that is larger than the overalltarget. In such overfilled mode, it may also be possible to measuredifferent targets simultaneously, thus determining different processingparameters at the same time. An example of such multiple targetmeasurements can be found in US patent application US20120123581A1,incorporated herein by reference in its entirety.

Overall measurement quality of a lithographic parameter using a specifictarget is at least partially determined by the measurement recipe usedto measure this lithographic parameter. The term “substrate measurementrecipe” may include one or more parameters of the measurement itself,one or more parameters of the one or more patterns measured, or both.For example, if the measurement used in a substrate measurement recipeis a diffraction-based optical measurement, one or more of theparameters of the measurement may include the wavelength of theradiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US20160161863A1 and not yet publishedUS patent application incorporated herein by reference in theirentirety.

FIG. 4 depicts a schematic representation of an embodiment of ametrology apparatus 400. The metrology apparatus may be metrologyapparatus MT of FIG. 3 . In this document the terms metrology apparatusis interchangeable with the term inspection apparatus.

Metrology apparatus 400 is for determining a characteristic of interestrelating to at least one structure STR, 460 on a substrate SUB, 450. Thecharacteristic of interest may be determining one or more values inrelation to the structure 460 on the substrate 450. The characteristicof interest may also be a deviation of the structure 460 on thesubstrate 450 from an expected structure. The characteristic of interestmay also be the presence or absence of a structure 460 on the substrate450 in comparison to an expected absence or presence of such astructure. The characteristic of interest may also be determining anorientation of the structure 460 on the substrate 450, or determining atilt of a surface of the structure 460, etc.

The metrology apparatus comprises a sensor SNS, 440 and an opticalsystem 420. The sensor 440 is for detecting a characteristic ofradiation being transmitted onto the sensor by the optical system 420.The sensor 440 may comprise an array of pixels that are capable ofgenerating an image of the radiation that impinges on the sensor 440.The sensor 440 may be arranged in an image plane of the optical systemor in a conjugate plane to the image plane. The sensor 440 may beconfigured to record an image of impinging radiation. The sensor 440 mayalso be configured to record images having a signal to noise ratio thatis being higher than 0.1, or, optionally, higher than 1, or optionally,higher than 10. The sensor may comprise a CCD camera that has anintegration time that is shorter than 1 second and, optionally, has anintegration time that is shorter than 500 μs. An acquisition time of thesensor 440 may comprises at least the integration time and may alsocomprise a time interval for signal processing and/or transmitting theobtained image to, for example, a determining system 470. In anembodiment the sensor 440 is operable to acquire an image, i.e. havingan acquisition time, until the received signal is above a thresholdvalue. In general, the sensor 440 may be configured to obtain within amaximum amount of time an image with a high enough signal to noise ratiothat can be processed by a pattern recognition algorithm to detect inthe image a structure that is similar to an expected structure.

The sensor may also be a based on lock-in detection.

The metrology apparatus may also be configured to suppress noise bymodulating the radiation generated by the source 410 according to apredefined pattern, and, thus, modulating the radiation that impinges onthe structure 460. Subsequently, radiation that is transmitted towardsthe sensor 440 will, for the largest part, also modulate according tothe predefined pattern and it is expect that the optionally presentnoise is not modulating according to that pattern. Thereby the sensor440 and/or the determining system 470 is able to distinguish noise fromsignals that originate from the structure 460 on the substrate by onlyrecording and/or analyzing signals that have the predefined modulationpattern.

The optical system 420 comprises an illumination path and a detectionpath. The optical system 420 is configured to illuminate the at leastone structure with radiation received from the source SRC, 410 via theillumination path. The optical system 420 is configured to receiveradiation scattered by the at least one structure 460 and to transmitthe received radiation to the sensor 440 via the detection path.

The optical system 420 is configured to image the at least one structure460 on the substrate 450 onto the sensor 440. Optionally, the precisionand resolution of the optical system 420 and/or of the sensor 440 aresuch that features of the structure 460 can be distinguishedindividually in an image that is being formed on the sensor 440 and/orthat is recorded by the sensor 440. This may imply that, for example,the arrangement of the optical elements of the optical system 420 isprecise enough to image the features onto the sensor 440. This may implythat the optical elements of the optical system 420 are precise enough,e.g., they have a low aberration, such that the features can bedistinguished in the image that is recorded by the sensor 440. This mayimply that the resolution of the sensor 440 is large enough todistinguish the individual features of the structure 460 on the sensor440. Thus, there is an appropriate amount of pixels available on thesensor 440 and the pixels of the sensor 440 are sensitive enough. Thismay also imply that the optical system 420 magnifies or demagnifies theimage of the features of the structure 460 in such a way that individualfeatures of the structure are imaged on one or more pixels of the sensor440.

In an embodiment the optical system 420 comprises a high NA lens, or theoptical system 420 in its entirety has a high NA. The resolution of animaging system, such as the optical system 420, is proportional to thewavelength of the light being observed with the sensor 440 or generatedby source 410, and inversional proportional to the size of itsobjective, as indicated by the Abbe diffraction limit

${d = \frac{\lambda}{2{NA}}},$

wherein NA is the numerical aperture of the objective and d is theradius of the spot formed by the light focused with the optical system420. For the purpose of forming an image, the spot size may be such thatit is smaller than the individual features which are to imaged. In apreferred embodiment, the optical system 420 comprises a high NA lens.In an embodiment, the NA is 0.7. In an embodiment, the NA is 0.8. In anembodiment, the NA is 0.9. In an embodiment, the NA is higher than 0.95.In an embodiment, there are specific materials present between theoptical system 420 and the structure 460, the NA is higher than 1.Examples of the specific materials are gasses that may increase an NA ora fluid, such as water.

It is to be noted that a difference may be distinguished between theillumination numerical aperture and a detection numerical aperture. Theillumination numerical aperture is the numerical aperture of the beam ofillumination radiation that impinges on the structure 460 on thesubstrate 450. This illumination numerical aperture may be significantlysmaller than the numerical aperture that the lens 424 of the opticalsystem 420 that is closest to the structure 460 supports because theillumination beam may only use a portion of the lens 424 of the opticalsystem 420 that is closest to the structure. In an embodiment, theillumination numerical aperture is smaller than 0.5, or optionally,smaller than 0.2, or optionally, smaller than 0.1, or, optionally,smaller than 0.05. In an embodiment, the detection numerical aperture isas large as possible. The detection numerical aperture defines up tillwhich diffraction angle higher diffraction angles can be captured by theoptical system 420 and can be transmitted towards the sensor 440. Inprevious paragraphs, the discussed numerical aperture (NA) relates tothe detection numerical aperture and thus to a large extend to thenumerical aperture of the lens 424 of the optical system that is closestto the structure 460.

The metrology apparatus 400 is configured to prevent a transmission ofradiation of the 0^(th) diffraction order of the scattered radiationtowards the sensor 440. As indicated in FIG. 4 , a ray of radiationimpinges on the structure 460 at a perpendicular angle with respect tothe substrate 450. The structure 460 scatters the radiation resulting ina zeroth diffraction order into a direction towards the optical system420 at an angle perpendicular to the substrate 450 and a plus and minusfirst diffraction order into the direction of the optical system 420 andboth having an angle with respect to the zeroth diffraction order.Additional higher diffraction orders may travel into a direction awayfrom the substrate 450. For clarity these higher diffraction orders arenot drawn. In the optical system a blocking element 428 may be presentthat blocks the zeroth diffraction order and prevents that the radiationof the zeroth diffraction order travels towards the sensor 440

The optical system may operate in a first operational mode and in asecond operational mode. In the first operation mode the blockingelement is controllable in the a first position where the blockingelement blocks the transmission of the 0^(th) diffraction order of thescattered radiation towards the sensor 440. In the second operationalmode the blocking element is at another (second) location where it doesnot block the transmission of the 0^(th) order towards the sensor 440.

Optionally, the blocking element 428 is present in a pupil plane or in aconjugate plane to the pupil plane. An important effect of blocking thezeroth order is that the dc-level, which would be present in the imageon the sensor 440 if the zeroth order is not blocked, is significantlyreduced and thereby the image has a better contrast.

In the example of FIG. 4 the illumination path and the detection pathpartially overlap. The optical system 420 has a beam splitting element426 that reflects radiation originating from the source 410 towards thestructure 460 on the substrate 450 and that allows the transmission ofradiation scattered by the structure 460 towards the sensor 440.

The optical system 420 may comprise one or more lenses 422, 424. The oneor more lenses 422, 424 are configured to focus the radiation from thesource 410 in a spot on the structure 460 and wherein the one or morelenses 422, 424 are configured to transmit the radiation scattered bythe at least one structure 460 towards the sensor 440. Optionally, theone or more lenses 422, 424 are arranged to create an image of the atleast one structure 460 on the sensor 440.

The lenses should have a relatively high quality in order to image theindividual features of the structure 460 on the sensor. As indicatedpreviously this may imply that the one or more lenses (individually oras a combination of lenses) have an aberration that is smaller thanλ/20. Optionally, the aberration of the individual lenses or thecombination of lenses is smaller than λ/100, or, optionally smaller thanλ/500, or, optionally smaller than λ/1000.

It is to be noted that the example of the optical system 420 compriseslenses 422, 424. Embodiments of such optical system 420 are not limitedto the use of lenses 422, 424. The optical system 420 may also use(curved) mirrors. Mirrors can be used instead of lenses. Also acombination of one or more lenses and one or more mirrors can be used aswell.

The optical system may comprise an optics errors measuring unit, notshown in the figures In an embodiment, the optics errors measuring unitmay be an aberration sensor, such as a Shack-Hartmann sensor. Detectedaberration may be used by the determining system 470 to correct an imagethat is recorded by sensor 440 for the detected aberration.

The metrology system 400 may also comprise a determining system DTRM,470. The determining system 470 is configured to receive a signal fromthe sensor 440 and the signal represents an image that is recorded bythe sensor. The determining system 470 determines an overlay value onbasis of a displacement between features in a first layer of thesubstrate 450 and features in a second layer of the substrate 450. Thedisplacement between the features is determined in the image that isreceived from the sensor 440. If the overlay value deviates from anexpected value, the deviation is termed the overlay error. Often theterm overlay is used to indicated overlay error.

Metrology system 400 may also comprise one or more actuators 480 thatare directly or indirectly coupled to the substrate 450 and which allowthe movement of the substrate with respect the position whereillumination radiation impinges on the substrate. The movement is, forexample, in the x, y and z direction. The substrate may be asemiconductor wafer and the substrate may be provided on a wafer table.The position of the wafer table may be controllable in x, y, z directionand the wafer table may also be operable to rotate around a centralaxis.

Metrology system 400 may, if actuators 480 are present, be operable toilluminate the structure 460 with a spot of radiation while thesubstrate 450 with structure 460 moves with respect to the position ofthe spot of radiation. It is not necessary that (only) the substratemoves, it may also be that the optical system with sensor 440 and/orsource 410 moves with respect to the substrate. During the moving of thesubstrate 450 with structure 460, the spot of radiation illuminatesdifferent parts of the substrate 450 and, thus, also different parts ofthe structure 460. During the moving, the sensor with determining system470 may obtain or record different images of the structure. Thesubstrate and the different images may be used to reconstruct an overallimage of the structure and/or may be used to directly determine valuesfor the characteristics of interest (such as, for example, overlayvalues).

In an embodiment of the optical system 420, the blocking element 428 maybe moveable in dependence of a control signal. For example, the blockingelement 428 may be moved towards a second position 428′ by means of atranslating or rotating movement. At the second position 428′ theblocking element is not anymore blocking the 0^(th) diffraction orderand then a bright field image is created on the sensor 440. Thisembodiment enables a metrology apparatus 400 that is capable ofswitching between dark field and bright field imaging.

In FIG. 4 it has been shown that the spot of radiation impinges on asingle structure. In specific embodiments, two or more structures 460,460′ may be available on the substrate 450 at positions that are closeto each other. The two or more structures 460, 460′ may be adjacent toeach other, optionally with a small distance in between them. Theoptical system 420 may be configured to illuminate two or morestructures 460, 460′ simultaneously with a single spot of radiation.Consequently, the image recorded by the sensor 440 may also comprise animage of the two or more structure 460, 460′ and characteristics ofinterest can be determined on basis of the recorded image for bothstructures. For example, the two or more structure 460, 460′ havefeatures in different layers and thereby one can determine, for example,overlay values that relates to different pairs of layers. Thisembodiment enables a faster acquisition of characteristics of interestbecause only a single acquisition of an image is needed to determinemultiple characteristics of interest.

FIGS. 6 and 7 will be used to explain how the overlay value isdetermined. The structure 460 on the substrate 450 may be manufacturedon the substrate 450 by means of, for example, the lithographicapparatus of FIG. 1 . In the lithographic apparatus a patterning deviceMA is used to print the structure 460. The patterning device MA maycomprise a structure that is intended to print structure 460 on thesubstrate 450. In practical embodiments a first patterning device isused or a few patterning devices are used to print features in a firstlayer and a second patterning device is used or a few patterning devicesare used to print features in a second layer. In practical embodiments,the first, second or additional patterning devices that are being useddo not comprise exact copies of portions of structure 600, but comprisestructures that result under predefined manufacturing conditions inportions of structure 600.

Thus FIG. 6 represents a structure 600 that is present on the substrateif it is being manufactured under ideal manufacturing circumstances and,thus, if no overlay error is being present. The structure 600 comprisesfirst structure 612 in a first layer and second structure 622 in asecond layer. All structures in structure 600 that have the samehatching are manufactured in the same layer. There are horizontallyoriented structures and vertically oriented structured. For sake ofclarity a first discussion only focuses on the horizontally orientedstructures. It is further noted that horizontally and vertically aredefined with respect to the orientation of the figure and that, inpractical embodiments, the presented pattern may be present in arotated, translated and/or mirrored pattern on the substrate 450.

Structure 600 comprises a first area 610 and a second area 620. In thefirst area 610 a repetitive pattern of first features 612 is present. Inthe second area 620 a repetitive pattern of second features 622 ispresent. In the example of structure 600, the first area and the secondarea do not overlap and are adjacent to each other. As present at 650 or660, the first area and the second may partially overlap.

The pitch of the first feature 612 and the pitch of the second features622 are substantially equal to each other. However, it is to be notedthat this is not necessary. As long as, as will be discussedhereinafter, the determining system 470 is able to determine an overlayvalue from the image being recorded by the sensor 440, varying pitchescan be used as well. The pitch between the features 612, 622 alsoinfluences the angle at which higher diffraction orders are scatteredand, therefore, the used pitch or pitches must also be chosen wiselysuch that as most as possible the higher diffraction orders, or at leastas most as possible one higher diffraction order, is captured by theoptical system 420. Note that the angle at which the higher orders arepresent also depends on the wavelength of the radiation that is used toilluminate the structure 460 and, therefore, the combination between thewavelength and the pitch must be chosen wisely to enable the creation ofan image on the sensor 440.

In practical situations, structure 600 is not exactly manufactured onthe wafer as indicated in FIG. 6 . In practical embodiments, on thesubstrate 450, structure 600 may look like the structure presented inthe upper half of FIG. 7 . An image 700 is recorded by sensor 440 ofmetrology apparatus 400. The image is similar to the presentedstructures at the upper half of FIG. 7 . As can already be seen in image700, the first features 712 are displaced with respect to the secondfeatures 722, in particular, when being compared to the ideal situationof structure 600. For the horizontally oriented features 712, 722 aportion of image 700 is enlarged at the bottom left side of FIG. 7 .There it can be seen in more detail that first feature 712′ is displacedwith respect to second feature 722′ over a distance OVL_(y). OVL_(y) isthe overlay value in the y-direction (wherein x and y-direction aredefined with respect to the orientation of FIG. 7 ). Image 700 alsoshows that there is a displacement between the vertically orientedfeatures 742, 732. At the bottom right side of FIG. 7 one can see thedisplacement is indicated with OVL_(x) and the distance OVL_(x) is theoverlay value in the x-direction.

Although it has been suggested by FIG. 7 that one can determine anoverlay value based on analyzing a displacement of e.g. two oppositehorizontally oriented features 712, 722, 712′, 722′ or two verticallyoriented features 742, 732 that are adjacent to each other, in practicalembodiments more pairs of adjacent features having the same orientationare used. For example, for each pair of horizontally oriented featuresan overlay value can be determined and the determined values can beaveraged. For example, it is also possible to determine a first kind ofwave pattern that is in phase with horizontally features in a firstlayer and determine a second kind of wave pattern that is in phase thehorizontally features in a second layer and use the difference betweenthe phases of the first kind of wave pattern and the second kind of wavepattern as a basis for the overlay value.

Because overlay values of a few nanometers must be measured it isimportant that the optical system 420 of metrology apparatus 400 has avery high precisions and accuracy with respect to the imaging of thestructure 460 on the sensor 440. About no deviations from an idealoptical system can be accepted. As such, aberration deviations of thelenses 422, 424 of the optical system 420 and also to the optical system420 as a whole are preferably small. The determining system 470 receivesthe image that is recorded by the sensor 440.

This image is, for example, image 700. The determining system 470 mayalso have knowledge about the structure that is expected in the image(e.g. the ideal structure 600 of FIG. 6 ) and by means of, for example,pattern recognition this structure is detected in image 700. The patternrecognition technology applied by the determining system 470 may be suchthat it is capable to detect a structure similar to the ideal structure,e.g. structure 600 of FIG. 6 , in an image that comprises a relativelylarge amount of noise, or in an image in which the signal noise ratio isrelatively small, or in an image in which the dynamic range isrelatively small. The determining system 470 may also have knowledgewhere a transition of a first feature in a first layer to a secondfeature in a second layer may be expected. Based on this knowledge sucha transition may be recognized in image 700 and the overlay values maybe determined at those transitions. The determining system 470 may, forexample, implement an edge detection system for detecting edges of, forexample, structures 712′, 722′, 742, 732.

When returning to FIG. 4 , other optional features may be present in theoptical system 420. The optical system 420 may comprise a wavelengthfilter 434. By way of example the wavelength filter 434 is drawn inbetween the source 410 and the beam splitting element 426 and it is tobe noted that the wavelength filter 434 may also be arranged at anotherlocation. It is expected that the wavelength filter 434 is at leastpresent in the illumination path of the optical system 420. Thewavelength filter 434 is configured to only transmit radiation of aparticular wavelength or within a particular wavelength range. In afurther embodiment, the operation of the wavelength filter 434 may becontrollable in dependence of a wavelength control signal. It is also tobe noted that the source 410 may also have a similar wavelength filter.In another embodiment, the wavelength filter 434 may comprise aplurality of filters of which the combination of filters allow thetransmission of two or more wavelengths that are spaced apart. Specificmaterials are opaque for specific wavelengths. Therefore, the selectionof the wavelengths that are used to illuminate the structure 460 (whichare the wavelengths that are transmitted through the wavelength filter434) strongly depends on the materials that are used in the structure460 and whether the illumination radiation has to be transferred throughspecific layers of the structure 460. The wavelength filter 434 may beconfigured to only allow a narrow band of wavelengths to be transmittedthrough the wavelength filter 434, for example, the width of the narrowband of wavelengths is smaller than 20 nm, or smaller than 10 nm, orsmaller than 5 nm.

It may be advantageous to illuminate the structure with a certainwavelength or with radiation within a certain wavelength range. Thematerials of the substrate 450, and as such also the features of thestructure 460, may strongly influence the scattering of the impingingradiation. By selecting a specific wavelength or by selecting certainwavelengths one may obtain a better image on the sensor 440.Additionally, the wavelength of the illumination radiation may beselected such that, giving that the structure 460 has a repetitivestructure with a given pitch, at least one higher diffraction order canbe captured by the optical system 420 (e.g. by front lens 424) fortransmission towards the sensor 440. In other words, the illuminationwavelength and the pitch of the repetitive structure are tuned to eachother (and are tuned with respect to the numerical aperture (NA) of thedetection path of the optical system 420).

In a further embodiment, the optical system 420 comprises a firstpolarizer 430 being arranged in the illumination path of the opticalsystem 420. The first polarizer 430 is configured to allow thetransmission of radiation having a certain polarization. The firstpolarizer 430 may also be a controllable polarizer of which thetransmittable polarization can be controlled in dependence of apolarization control signal.

In an additional embodiment, the optical system 420 comprises a secondpolarizer 432 being arranged in the detection path of the optical system420. The second polarizer 432 is configured to allow the transmission ofradiation having a certain polarization. The second polarizer 432 mayalso be a controllable polarizer of which the transmittable polarizationcan be controlled in dependence of a further polarization controlsignal.

It may be advantageous to control the polarization of the radiation thatimpinges on the structure and to control which polarization of thescattered radiation is transmitted to the sensor. The materials of thesubstrate 450, and as such also of the structure 460, may scatterradiation of different polarizations in different ways. The structure460 and also the materials of the structure 460 and the substrate 450may change the polarization of the radiation that impinges on thestructure 460 and that is being scattered. Thereby one can obtain abetter image, e.g. with a higher contrast, on the sensor 440 byilluminating with a certain polarization and only allowing thetransmission of the same or another specific polarization towards thesensor. It has also been observed that a selection of specificwavelengths of radiation in combination with specific polarizations mayresult in a better image on the sensor 440. In an embodiment thestructure 460 is illuminated by radiation having a first polarization asdetermined by the first polarizer 430. With specific structures 460 itis known that the information that relates, for example, to overlay ispresent in radiation that has a polarization direction that isperpendicular to the first polarization. Then the second polarizer 432only allows the transmission of radiation having a polarizationdirection perpendicular to the polarization direction that istransmittable through the first polarizer 430. In an embodiment, asshown in FIG. 4 and if at least the second polarizer 432 is present,only higher order diffraction radiation of a certain polarizationdirection impinges on the sensor 440 and the determining of thecharacteristic of the structure 460 is only based on one or more higherorder diffraction radiation of a certain polarization direction. In anembodiment, the settings of the first polarizer 430 and/or of the secondpolarizer 432 are optimized for the specific structures 460 that arepresent on the substrate 450 to obtain a better image, e.g. with ahigher contrast, on the sensor 440. For example, if the specificstructures 460 has repetitive lines in a certain direction than it maybe that illumination radiation with a certain polarization is maximallyscattered (optionally, maximally scattered into the higher diffractionorders) and that only transmitting a specific (other) polarization ofthe scattered radiation towards the sensor may result in an image withmost contrast and/or a highest image quality.

FIG. 4 also shows the source 410. The source 410 may be part of themetrology apparatus 400 but may also be a separate source that providesradiation to the metrology apparatus 400 by means of, for example, glassfibers and/or another type of light guide. The source is configured togenerate radiation having one or more wavelengths in a wavelength rangefrom 200 nm to 2000 nm, or optionally, in a range from 300 nm to 1000nm, or, optionally, in a range from 400 nm to 900 nm, or, optionally, ina range from 400 nm to 700 nm. The source may be configured to generateradiation in the near-infrared spectral range, for example, from 750 nmto 1400 nm. The source may be configured to generate radiation in thevisible wavelength range, for example, from 380 nm to 750 nm. Forexample, the source may be configured to generation radiation in atleast one of the Ultraviolet A, B or C spectral ranges, respectively,from 315 nm to 400 nm, from 280 nm to 315 nm, from 100 nm to 280 nm. Asindicated above, radiation of specific wavelengths may be beneficial andas such the wavelength or the wavelengths of the radiation may beoptimized for obtaining an image of a high enough quality on the sensor440. The source may also be configured to emit radiation at two or morewavelengths that are spaced apart.

The amount of power being present in the radiation that illuminates thestructure 460 determines for a large amount the time required forrecording an image by the sensor. The more power present in theradiation that illuminates the structure 460, the more radiationimpinges on the sensor and the shorter the integration time is that thesensor 440 needs to record an image. As such, the source 410 isconfigured to generate in use a radiation that has a power larger than50 Watt, or, optionally, larger than 150 Watt, or, optionally, largerthan 250 Watt, or, optionally, larger than 1000 Watt.

In an embodiment, the metrology apparatus 400 may use plasma-basedphoton sources, for example laser driven photon sources (LDPS),otherwise known as laser-driven light sources, as these offer highbrightness. Plasmas are generated in a gaseous medium by the applicationof energy through electric discharge, and laser energy. The spectraldistribution of the radiation may be broadband or narrowband in nature,and wavelengths may be in the near infrared, visible and/or ultraviolet(UV) bands. Published patent application US2011204265 and internationalpatent application WO2016030485 disclose plasma-based light sourcesincluding laser-driven photon sources, which documents are incorporatedherein by reference in their entirety.

Other examples of sources are a coherent white light laser, a coherentdiscrete laser (which emits within a small wavelength range), a coherentcontinuous controllable laser, a coherent optical parametric oscillator(OPO), an incoherent laser driven light source (as discussed, forexample, above), and/or an incoherent photodiode. It is to be noted thatpartially incoherent sources may be used as well.

It is to be noted that the source 410 may also be a controllable sourcethat is capable of emitting radiation of a controllable wavelengthwithin a wavelength range in dependence of a source control signal.

The source 410 may emit radiation in a relatively wide wavelength range,for example, in a wavelength range that is wider than 50 nm, or,optionally, in a wavelength range that is wider than 100 nm, or,optionally, in a wavelength range that is wider than 200 nm. The source410 may also be configured to emit in a relatively narrow wavelengthrange, wherein the width of the narrow wavelength range is, for example,smaller than 20 nm, or smaller than 10 nm, or smaller than 5 nm.

In FIG. 4 only one illumination radiation ray is drawn. Schematicallyseen this single radiation ray is a central axis of a radiation beamthat is transmitted and focused in a spot of a certain size on thestructure 460. As such, the drawn −1^(st), +1^(st) and 0^(th)diffraction orders are also beams of radiation of which only a schematiccentral axis is drawn. In the context of FIG. 4 , the central axis ofthe illumination radiation beam impinges on the structure 460 at adirection substantially perpendicular to the top surface of thesubstrate.

A metrology recipe can be used that specifies one or more parameters ofthe measurement using an embodiment of the metrology apparatus asdiscussed in this application. In an embodiment, the term “metrologyrecipe” includes one or more parameters of the measurement/of themetrology apparatus itself, one or more parameters of a structure, e.g.structure 460, measured, or both.

In this context, a structure 460 measured (also referred to as a“target” or “target structure”) may be a pattern that is opticallymeasured, e.g., whose diffraction is measured or which is imaged on, forexample, sensor 440. The pattern measured may be a pattern speciallydesigned or selected for measurement purposes. Multiple copies of atarget may be placed on many places on the substrate 450. For example, ametrology recipe may be used to measure overlay. In an embodiment, ametrology recipe may be used to measure another process parameter (e.g.,dose, focus, CD, etc.). In an embodiment, a metrology recipe may be usedfor measuring alignment of a layer of a pattern being imaged against anexisting pattern on a substrate; for example, a metrology recipe may beused to align the patterning device to the substrate, by measuring arelative position of the substrate.

In an embodiment, if the metrology recipe comprises one or moreparameters of the measurement/of the metrology apparatus 400 itself, theone or more parameters of the measurement itself can include one or moreparameters relating to an illumination beam and/or metrology apparatusused to make the measurement. For example, the one or more parameters ofthe measurement itself may include a wavelength of illuminationradiation, and/or a polarization of illumination radiation, and/orillumination radiation intensity distribution, and/or an illuminationangle (e.g., incident angle, azimuth angle, etc.) relative to thesubstrate 450 of illumination radiation, and/or the relative orientationrelative to a structure 460 on the substrate 450 of diffracted/scatteredradiation, and/or the number of measured points or instances of thestructure 460/target, and/or the locations of instances of the structure460/target measured on the substrate 450. The one or more parameters ofthe measurement itself may include one or more parameters of themetrology apparatus used in the measurement, which can include detectorsensitivity, numerical aperture, etc.

In an embodiment, if the metrology recipe comprises one or moreparameters of a structure 460 measured, the one or more parameters ofthe structure 460 measured may include one or more geometriccharacteristics (such as a shape of at least part of the structure 460,and/or orientation of at least part of the structure 460, and/or a pitchof at least part of the structure 460 (e.g., a pitch of a gratingincluding the pitch of an upper grating in an upper layer, of a lowergrating and/or the pitch of the lower grating), and/or a size (e.g., CD)of at least part of the structure 460 (e.g., the CD of a feature of agrating, including that of a feature of the upper grating and/or thelower grating), and/or a segmentation of a feature of the structure 460(e.g., a division of a feature of a grating into sub-structures), and/ora length of a grating or of a feature of the grating), and/or amaterials property (e.g., refractive index, extinction coefficient,material type, etc.) of at least part of the structure 460, and/or anidentification of the structure 460 (e.g., distinguishing a patternbeing from another pattern), etc.

A metrology recipe may be expressed in a form like (r₁, r₂, . . . r₃,r_(n); t₁, t₂, t₃, . . . t_(n)) where r₁ are one or more parameters ofthe measurement/of the metrology apparatus 400 and t₁ are one or moreparameters of one or more structure 460 measured. As will beappreciated, n and m can be 1. Further, the metrology recipe does notneed to have both one or more parameters of the measurement and one ormore parameters of one or more patterns measured; it can have just oneor more parameters of the measurement or have just one or moreparameters of one or more patterns measured.

In order to create a relatively good image on the sensor 440, themetrology apparatus 400 may focus the illumination radiation on thestructure 460 on the substrate 450. A focusing sub-system may beprovided in metrology apparatus 400. In an embodiment, the metrologyapparatus 400 may be configured to focus at different depths within thestructure 460. The optical system 420 may be arranged to move, forexample, lens 424 to obtain a specific focus. The one or more actuators480 that may move the substrate 450 in different directions may move thesubstrate 450 with structure 460 up and down to focus the illuminationradiation at different depths in the structure 460. The metrologyapparatus 400 may be configured to record with the sensor 440 differentimages when the metrology apparatus 400 is focused at different depthswithin the structure 460. The different images may be used by thedetermining system 470 to determine the characteristic of interest ofthe structure 460. The determining system 470 may also select one ormore images of the different images that are most suitable fordetermining the characteristic of interest. For example, the one or moreimages may be selected on basis of a requirement that the contrast mustbe above a predefined level.

FIG. 5 depicts a schematic representation of another embodiment of ametrology apparatus 500. Metrology apparatus 500 is similar to metrologyapparatus 400 and comprises similar or equal embodiments. Differenceswill be discussed hereinafter.

In metrology apparatus 500 the illumination radiation beam does notimpinge on the structure 460 at an angle that is substantiallyperpendicular to the structure 460, but impinges on a certain angle withrespect to a normal of the surface of the substrate 450. In the exampleof FIG. 5 this may result in a −1^(st) diffraction order that follows apath that is about perpendicular to the top surface of the substrate450, and a −2^(nd) and 0^(th) diffraction order that each have an anglewith respect to the normal. In such a configuration, the blockingelement 528 must be positioned at another location within the opticalsystem 520 such that the 0^(th) diffraction order is not transmittedtowards the sensor 440. Optionally, further blocking elements 528′ maybe present to block the −2^(nd) diffraction order as well. The opticalsystem 520 may also have an circular aperture in the detection path andblocking elements 528, 528′ may be a cross sectional view of the elementin which the aperture is created.

An illumination radiation beam that does not impinge perpendicular onthe structure 460 may be obtained by displacing the source 410 withrespect to a central axis of the illumination path of the optical system520. If the source emits a relatively wide radiation beam, one may placean illumination pupil 529 in the radiation beam that originates from thesource and that has an aperture at a location that is away from thecentral axis of the illumination path of the optical system 520. In anembodiment, the illumination pupil 529 may be controllable wherein theposition of the aperture with respect to the central axis of theillumination path of the optical system 520 can be controlled independence of an illumination pupil control signal.

FIG. 12 schematically shows exemplary top views of illumination pupils1200 and 1210 that can be used at the position of illumination pupil529. In illumination pupil 1200 an opaque plate 1202 has a hole 1204 atan off-center position. If a center of the opaque plate 1202 coincideswith the optical axis of the illumination path, the illuminationradiation will be a radiation beam that has, for example, the drawnillumination light ray of FIG. 5 as a center. Of course the illuminationpupil 1200 may also have two holes 1204 in a similar configuration asshown in illumination pupil 1210.

Illumination pupil 1210 comprises an opaque plate 1212 in which twoholes 1214, 1216 are made with a shape as shown in FIG. 12 . Thesespecific holes 1214, 1216 have, with respect to a center of theillumination pupil 1210, an angular offset of 90 degrees. This allowsthe transmission of two illumination beams that do not coincide with thecentral optical axis of the illumination path. This results in theillumination of the structure 460 from two different orthogonaldirections and may have the advantage that overlay values in twodimensions may be measured in one measurement acquisition.

In FIG. 5 another type of pupil, detection pupil 528″, has beenschematically drawn along the detection path of metrology apparatus 500.For example, detection pupil 528″ may be provided in a pupil plane or aplane conjugate with the pupil plane. The detection pupil 528″ may bearranged to allow the transmission of one or more higher diffractionorder of the scattered radiation and may be arranged to block thetransmission of the 0^(th) diffraction order. The detection pupil 528″may be used instead of or in addition to the blocking element 528. Forexample, in FIG. 12 an exemplary top view of an detection pupil 1220 isshown that comprises an opaque plate 1222 and a hole 1234 through which,if used for example in the configuration of FIG. 5 , higher diffractionorders can be transmitted and the 0^(th) order can be blocked. Analternative configuration is shown for detection pupil 1230 whichcomprises an opaque plate 1232 and two holes 1234 and 1236. It is to benoted that the skilled person may find alternative embodiments thathave, in combination with for example a specific illumination pupil, aspecific advantage with respect to allowing the transmission of certainhigher diffraction orders and blocking the 0^(th) diffraction order.

The illumination pupil 529 or detection pupil 528″ may also be formed byflexible and/or controllable types of pupils. Examples are provided inFIGS. 13(a) and 13(b) and FIGS. 14(a) and 14(b). The examples of FIGS.13(a), 13(b), 14(a) and 14(b) may be used in the examples of themetrology apparatus 400, 500 e.g. in the illumination path and/or in thedetection path. Possible locations in the apparatus are schematicallyindicated by illumination pupil 529 in metrology apparatus 500 or by thedetection pupil 528″ in metrology apparatus 500. It is to be noted thatthe examples of FIGS. 13(a), 13(b) and 14(a) are based on transmissionof radiation and as such the indicated positions in FIG. 5 are directlysuitable for these examples. The example of FIG. 14(b) is based onreflection and, as such, the metrology apparatus must be modified tohave an illumination and/or detection path which is reflected at theillumination and/or detection pupil. All example of FIGS. 13(a), 13(b),14(a) and 14(b) are examples of flexible pupil in which certain elementsare controllable to obtain a specific shape of the pupil. The examplesallow the creation of different specific shapes that may be selected independence of the requirements of a specific measurement.

FIG. 13(a) schematically shows a flexible pupil arrangement 1300 inwhich there are two wheels 1300, 1320 with differently shaped apertures1311 . . . 1314, 1321 . . . 1324 in the opaque material of the wheels.In use, wheel 1320 is located in front of or behind wheel 1310 at aposition that is indicated with dashed circle 1320′. The wheels 1300,1320 can be rotated in dependence of a control signal. By selecting aspecific position for the first wheel 1310 and a specific position forthe second wheel, a specifically shaped aperture is created by theflexible pupil arrangement 1300. In the example of FIG. 13(a), in use,it may be that aperture 1321 is in front of aperture 1314. In use, theposition of aperture 1321 is schematically shown by means of rectangle1321′. Seen in a direction perpendicular to the plane of the figure, theconjunction of aperture 1314 and aperture 1321′ is the specificallyselected shape of the pupil that is created in this example. It is to benoted that in the example of FIG. 13(a) only a limited number ofapertures are provided with a limited amount of different shapes. Inpractical embodiments, the wheels may have much more apertures with alarger variety of shapes thereby providing more control over thespecific shape of the created pupil.

FIG. 13(b) schematically shows a flexible pupil arrangement 1330. In sofar applicable, discussed characteristics of the flexible pupilarrangement 1300 of FIG. 13(a) also apply to the flexible pupilarrangement 1330 of FIG. 13(b). Flexible pupil arrangement 1330comprises two moveable strips 1340, 1350 of an opaque material and themoveable strips 1340, 1350 comprise different apertures 1341 . . . 1346,1451 . . . 1354 of a different shape and/or size. By moving a specificaperture 1341 . . . 1346 of the first strip 1340 behind or in front of aspecific aperture 1351 . . . 1354 of the second strip 1350, a pupil ofspecific controllable shape can be created. In the example of FIG.13(b), in the viewing direction of the reader, the second strip 1350 isplaced in front of the first strip 1340 and aperture 1351 is in front ofaperture 1345. Seen in a direction perpendicular to the plane of afigure, the conjunction of the apertures 1351, 1345 defines the shape ofthe pupil.

FIG. 14(a) schematically shows a third embodiment of a flexible pupilarrangement 1400. In so far applicable, discussed characteristics of theflexible pupil arrangement 1300 of FIG. 13(a) and the flexible pupilarrangement of 1330 of FIG. 13 (b) also apply to the flexible pupilarrangement 1400 of FIG. 14(a). The flexible pupil arrangement 1400comprises a plate 1410 or another shape of an opaque material and in theplate is provided an aperture 1420 that defines the largest possibleaperture that can be created with the flexible pupil arrangement 1400.The flexible pupil arrangement 1400 also comprises radiation blockingelements 1440, 1441, 1442 that are moveable along one of a multiple ofguidance structures 1430. On basis of a control signal, one or more ofthe radiation blocking elements 1440, 1441, 1442 may move along one ofthe guidance structures 1430 to a specific position in front of theaperture 1420 or not in front of the aperture 1420. By placing specificradiation blocking elements 1441 at specific positions in front of theaperture 1420, a portion of the aperture cannot transmit radiation andthe effective remaining aperture is reshaped towards a smaller apertureof a specific shape. The radiation blocking elements 1440, 1441, 1442may be box shaped elements of an opaque material. In another embodiment,the radiation blocking element 1440, 1441, 1442 may have a spherical orellipsoid shape, or may be relatively thin plate shaped elements thatare arranged parallel to the plate 1410. The radiation blocking elements1440, 1441, 1442 may be shaped such that they partially overlay whenthey are at moved close to each other e.g. in front of the aperture1420. E.g. the radiation blocking elements 1440, 1441, 1442 may bestrips that are arranged in a tilted configuration with respect to theplane defined by the plate 1410. The guidance structures 1430 may bethin wires, or thin strips that are arranged parallel to the planedefined by the plate 1410. The radiation blocking elements 1440, 1441,1442 may be moved by means of mechanical forces (e.g. by means ofactuators), or by electrical or magnetic forces.

FIG. 14(b) schematically presents a fourth embodiment of a flexiblepupil arrangement 1450. In so far applicable, discussed characteristicsof previously discussed flexible pupil arrangement 1300, 1330, 1400 mayalso apply to the flexible pupil arrangement 1450 of FIG. 14(b). Theflexible pupil arrangement 1450 comprises an array of controllable micromirrors 1451 . . . 1456 provided on a supporting structure 1470. In thearray of controllable micro mirrors 1451 . . . 1456 groups of micromirrors 1451 . . . 1456 and/or each individual micro mirror 1451 . . .1456 is or are controllable in such a way that impinging radiation 1460may be reflected towards a specific direction, e.g. a first direction d1or a second direction d2. In an embodiment, the micro mirrors may beconfigured to controllably reflect impinging radiation in more than twodirections. In the example of FIG. 14(b) some radiation rays 1460 (e.g.light rays) are schematically indicated. The radiation rays 1460 mayoriginate from a source if the flexible pupil arrangement 1450 isprovided in the illumination path. In the example of FIG. 14(b), micromirrors 1452, 1456 are controlled in such a way that impinging radiationis reflected in the second direction d2 and micro mirrors 1453 . . .1455 are controlled in such a way that impinging radiation is reflectedin the first direction d1. If the flexible pupil arrangement 1450 isprovided in the illumination path, and if the light that is reflected inthe first direction d1 is used for illumination, then the flexible pupilarrangement 1450 has a (virtual) pupil that is defined by all micromirrors 1453 . . . 1455 that reflect light into the first direction d1.

It is to be noted that the embodiment of FIG. 14(b) is also often calleda spatial light modulator. Other embodiments of spatial light modulatorsmay also be used a flexible pupil arrangements in the metrologyapparatuses 400, 500. Another embodiment of such a spatial lightmodulator is a kind of Liquid-Crystal Display (LCD) that has an array ofpixels that may be controlled in a transmission state or in a radiationblocking state or in a reflection state. An LCD spatial light modulatormay be used as a flexible and/or controllable pupil in transmission modeor in reflection mode.

FIG. 6 depicts a number of schematic representations of structures thatmay be manufactured on a substrate and that may be inspected by theembodiments of the metrology apparatus. Structure 600 has already beendiscussed above. Structure 650 is a structure in which the first area610 and the second area 620 partially overlap and are displaced withrespect to each other in the y-dimension. Rotations of structure 650 mayalso form a larger structure that is, for example, similar to structure600. Structure 660 is similar to structure 650 with a difference thatthere is no displacement in the y-dimension.

As discussed above, image 700 of FIG. 7 may be processed by adetermining system 470 of the metrology apparatus 400. The determiningsystem 470 may be configured to detect a region of interest in the image700. Regions of interest are regions in the image that are used forfurther processing and that are used to determine, for example, anoverlay value or another characteristic of the structure 460 on thesubstrate 450. For example, region 790 is identified as a region ofinterest because it may comprise a predefined pattern or at least apredefined pattern that is similar to an expected pattern. Thedetermining system 470 may be further configured to determine a smallerregion of interest for an analysis. For example, as shown at the bottomleft side of FIG. 7 , a further region of interest 790′ may bedetermined within the image. In this example, the further region ofinterest 790′ is a region of interest for determining an overlay valuein the y-dimension. In yet another example, as shown at the bottom rightside of FIG. 7 , another further region of interest 790″ is a region ofinterest for determining an overlay value in the x-dimension. It is tobe noted that if the image on the sensor 440 is only based on higherdiffraction orders (higher than 0^(th) diffraction orders) then theregions of interest 790, 790′, 790″ are only based on radiation in thehigher diffraction orders of the radiation that is scattered by thestructure 460. If, as discussed for example in the context of FIG. 4 ,the blocking element 428 may be controllable and if it is controllablewhether the 0^(th) diffraction order impinges on the sensor 440, thenthe regions of interest 790, 790′, 790″ may also be determined on basisof an image that is the result of impinging radiation of the 0^(th) andhigher diffraction orders.

FIG. 8 depicts a schematic representation of a third embodiment of ametrology apparatus 800. Metrology apparatus 800 is similar to metrologyapparatuses 400 and 500 of FIGS. 4 and 5 . Differences will be discussedhereinafter. Embodiments of metrology apparatuses 400 and 500 may becombined with the metrology apparatus 800.

Metrology apparatus 800 has a different optical system. Instead of thebeam splitter 426 a mirror 828 is provided. The mirror 828 is configuredto reflect the radiation originating from the source 410 towards thestructure 460 on the substrate 450 at an angle substantiallyperpendicular to the top surface of the substrate 450. The specular(0^(th)) diffraction order that returns from the structure follows alsoa path that is substantially perpendicular to the top surface of thesubstrate 450 and also impinges on the mirror 828 and is reflected backtowards the source. More details about such a mirror 828 or similarelements in an alignment tool can be found in WO2014/026819A2 (e.g. FIG.3 , mirror 223), WO2014/056708A2, WO2014/068116A1, WO2013/152878A2,which are incorporated herein by reference in their entirety. It is tobe noted that, in a configuration that is similar to the metrologyapparatus 500 of FIG. 5 , the blocking element 528 may be replaced by amirror that reflects the 0^(th) order in a direction that is not towardsthe sensor 440. Optionally, if the metrology apparatus 500 of FIG. 5 hassuch a mirror instead of the blocking element 528, the 0^(th)diffraction order can be reflected towards an additional sensor that canbe used to record information of the 0^(th) order. More examples of therecording of information of the 0^(th) order are given in the context ofFIG. 9 .

FIG. 9 depicts a schematic representation of a fourth embodiment of ametrology apparatus. Metrology apparatus 900 is similar to metrologyapparatuses 400, 500, 800 of FIGS. 4, 5 and 8 . Differences will bediscussed hereinafter. Embodiments of metrology apparatuses 400, 500 and800 may be combined with the metrology apparatus 900.

The optical system 920 is different from optical system 420 of metrologyapparatus 400. Instead of a blocking element 428 a mirror 928 isprovided which reflects the 0^(th) diffraction order into a directionthat is not towards the sensor 940. In an embodiment, the 0^(th)diffraction order is reflected towards a further sensor 942.

The further sensor 942 may be an intensity sensor to sense the intensityof the radiation in the 0^(th) diffraction order. This may be used as areference signal that can be used to normalize, for example, intensitiesof radiation that are measured with the sensor 440. Thus, the furthersensor 942 may have a diagnostics function with respect to, for example,the operation of the source.

The further sensor 942 may be arranged in the pupil plane or a conjugateplane with the pupil plane. Then a pupil image of the 0^(th) diffractionorder may be recorded and this pupil image may be used as an additionalsource of input to determine the characteristic of interest of thestructure 460. The pupil images can be used in a reconstruction processto determine the geometrical structure of structure 460 on the substrate450. It is also know that there may be, depending on the structure 460,overlay value information available in the pupil image.

In a further embodiment of the optical system 920 there is also afurther lens 926 in a path from the mirror 928 to the further sensor 942and the further lens 926 forms a bright field image of the structure 460on the sensor 942. The further sensor 942 may be configured to recordthe bright field image. The bright field image may be used for referencepurposes, or for alignment purposes, or for roughly estimating overlayvalues, etc. For example, a rough overlay value is determined in thebright field image based on techniques that are known in the art (asdiscussed in one or more of the incorporated documents). The roughoverlay value may be used to decide whether a process is out of spec.The rough overlay value may also be used to decide whether a moreprecise value must be determined on basis of the (dark field) image thatis formed on sensor 440. The further sensor 942 may also be coupled tothe determining system 470 or may be coupled to a further determiningsystem. If the further sensor 942 is used for alignment purposes, thedetermining system would try to recognize the expected structure in therecorded image to determine whether the structure is within the spot ofillumination. The determining system may be coupled to actuators thatare operable to move the substrate and then the metrology apparatus isoperable to search for the structure by an iterative move and detectalgorithm.

The further sensor 942 may further cooperate with the determining system470 for detecting a region of interest, for example, the region ofinterest 790, 790′, 790″ of FIG. 7 .

The information recorded by the further sensor 942, which is either apupil image of the 0^(th) diffraction order or an image of the structure460 based on the 0^(th) diffraction image, may be used to detectcharacteristics of a part of the optical system 920. For example,aberrations may be detected and the detected aberrations may be used by,for example, the determining system 470 to correct the image recorded bysensor 440. There may also be a reference structure on the substrate 450or there may be a fiducial target on the wafer table that supports thesubstrate 450 that is designed to allow a good detection ofcharacteristics of the optical system 920, such as aberration of opticalelements of the optical system 920.

Optionally, a position of mirror 928 is controllable. At a firstposition mirror 928 reflects the 0^(th) diffraction order towards, forexample, sensor 942. At a second position mirror 928′ does not block thetransmission of the 0^(th) diffraction order towards sensor 440. Such acontrollable mirror enables that the metrology apparatus 900 is capableto switch between dark field imaging and bright field imaging. In FIG. 9it is suggested that the mirror 928 is capable of rotating around anaxis. Alternatively mirror 928 is moved by means of a translationtowards a position completely outside the detection path of the opticalsystem 920.

FIG. 10 depicts schematic representations of structures to bemanufactured on the substrate to determine overlay values.

At the top left of FIG. 10 is presented a box in box structure 1000 thatmay comprise a first structure 1002 in a first layer and that maycomprise a second structure in a second layer 1004. The first structure1002 and the second structure 1004 may be a square or, alternatively,they may be rectangularly shaped. Even in other embodiment they may betriangular, circular or elliptical. The presented structure 1000 is thestructure that may be manufactured under ideal manufacturing conditionswith use of, for example, the lithographic apparatus of FIG. 1 . Theideal manufacturing conditions relate, for example, to the absence of anoverlay error. It must also be noted that, in practical embodiments, themanufactured structures 1002, 1004 on the substrate 450 may have roundcorners. In structure 1000 also a dot is presented that indicates thecenter of gravity 1003 of the first structure 1002 and the center ofgravity 1005 of the second structure 1004. Because in structure 1000there is no overlay error, the centers of gravity 1003, 1005 are on topof each other. For sake of clarity it is indicated that the dots thatrepresent the centers of gravity are not present in the structure 460 onthe substrate 450 but are only drawn in the figure to schematicallyindicate the centers of gravity. In practical embodiments thedetermining system searches in the recorded image for the center ofgravity by known methods or algorithms.

As presented at structure 1010, there may be an overlay error in themanufacturing process and that is detectable by means of a displacementof the second structure 1004′ with respect to the first structure 1002′,and, thus, also a displacement of the center of gravity 1003′ of thefirst structure 1002′ with respect to the center of gravity 1005′ of thesecond structure 1004′. A distance between the centers of gravity 1003′,1005′ in the x-dimension is the overlay value OVL_(x). A distancebetween the centers of gravity 1003′, 1005′ in the y-dimension is theoverlay value OVL_(y). In another embodiment, the determining system 470is operable to find corners of the structures 1002′, 1004′ by means ofpattern recognition and a displacement between the corners may be abasis for determining the overlay value(s).

Structure 1020 represents another structure that can be used todetermine overlay values. The structure 1020 is a bar in bar structureand comprises elongated rectangular first structures 1022 in a firstlayer and comprises elongated rectangular second structures 1024 in asecond layer. All structures that have the same hatching are to bemanufactured in the same layer. The structure 1020 as presented ismanufactured under ideal manufacturing conditions. It is to be notedthat, in practical embodiments, the corners of the bars 1022, 1024 maybe rounded and that line ends may become shortened. An overlay value inthe x-dimension can be detected by analyzing displacements of thevertical bars and an overlay value in the y-dimension can be detected byanalyzing displacements of the horizontal bars. In line with the exampleof structures 1000, 1010 an overlay value can also be determined bydetecting a displacement of a center of gravity of the first structures1022 with respect to a center of gravity of the second structure 1024.

Optionally, on a printing device, such as for example a reticle, that isused to print/manufacture the second structures 1024, sub-resolutionassist features 1025, 1025′ may be present to assist the printing of thesecond structure 1024. Please note that sub-resolution assist features1025, 1025′ do, in general, not print on the substrate 450 and they arenot present in the structure 460 on the substrate 450.

Structure 1030 comprises several cross-shaped structures 1032, 1032′,1034, 1034′. The first cross-shaped structure 1032, 1032′ are to bemanufactured in a first layer and the second cross-shaped structures1034, 1034′ are to be manufactured in a second layer. All structuresthat have the same hatching are to be manufactured in the same layer.The structure as presented is the structure that has to appear on thesubstrate 450 if the manufacturing conditions are ideal. It is to benoted that, in practical embodiments, the corners of the structures1032, 1032′, 1034, 1034′ may be rounded and that line ends may becomeshortened. If the manufacturing conditions are not ideal, the firstcross-shaped structures 1032, 1032′ are displaced with respect to thesecond cross-shaped structures 1034, 1034′. The overlay values in thedifferent dimensions can be obtained to analyze a specific displacementbetween two or more of the cross-shaped structures 1032, 1032′, 1034,1034′. For example, an overlay value in the y-dimension can be easilydetermined by determining a displacement distance of the firstcross-shaped structure 1032 with respect to the second cross-shapedstructure 1034. For example, an overlay value in the x-dimension can beeasily determined by determining a displacement distance of the firstcross-shaped structure 1032′ with respect to the second cross-shapedstructure 1034′. In the context of structure 1030 it is also to be notedthat patterning devices, such as reticles, that are used to manufacturestructure 1030, may comprise sub-resolution assist features that assistin the printing of first and second cross-shaped structures 1032, 1032′,1034, 1034′. Features similar to 1025, 1025′ may be present close to thelines of the of first and second cross-shaped structures 1032, 1032′,1034, 1034′.

Other multi-layer structures that can be used for determining overlayvalues are disclosed, for example, in FIG. 4 of “Diffraction ordercontrol in overlay metrology: a review of the roadmap options”, MikeAdel, et al, “Metrology, Inspection, and Process Control forMicrolithography XXII”, Proceedings of SPIE, Vol. 6922, paper 692202,2008, which is hereby incorporated by reference in its entirety.

FIG. 11 depicts schematic representations of sub-segmentations ofembodiments of targets. At the top of FIG. 11 is presented a possibletarget 1100 that may be used to determine overlay between differentlayers. The picture that is shown of target 1100 is the picture that is,under ideal circumstances, visible as an image on the sensor 440 ofpreviously discussed metrology apparatuses. The structure 460 onsubstrate 450 may be different and depending on the illumination anddetection characteristics, one may obtain the image of target 1100 thatis presented at the top of FIG. 11 . In line with for example FIGS. 6, 7and features drawn with different hatching relate to features indifferent layers of the structure 460 on substrate 450.

In general the idea is that the structures 460 on the substrate 450 mayhave, for the features that are visible (and/or invisible as a “space”),a number of smaller features. This is often termed “sub segmentation ofthe features”. As will be discussed later, areas that are presented intarget 1100 as “spaces” may have relatively small features that are, forexample, not visible on the sensor 440 as individual small features(e.g. each individual line may not be visible on the image recorded bythe sensor 440). Sub segmentation may be used for influencing thecontrast between different portions of the target 1100. Thus, the subsegmentation may have influence on the image on the sensor 440.

A first embodiment of sub segmentation 1120 is presented at the middleleft side of FIG. 11 . It is shown that for example features 1112, 1112′of a target are present on the substrate as dense configurations 1122,1122′ of lines at a relatively small pitch. The spaces between features1112, 1112′ of target 1100 may still have configurations 1123, 1123′ ofa few relatively narrow lines at a larger pitch than the pitch ofconfigurations 1122, 1122′. The configurations 1123, 1123′ scatter lightin a different manner than the configuration 1122, 1122′ and are,therefore, imaged in a different way on the sensor 440.

A second embodiment of sub segmentation 1130 is presented at the bottomof FIG. 11 . The features 1112, 1112′ of target 1100 may be present onthe substrate as configurations 1132, 1132′ that have a dense pattern ofvertical lines at a certain pitch. The spaces between features 1112,1112′ of target 1100 may be present on the substrate as configurations1133, 1133′ that have a dense pattern of horizontal lines at a certainpitch. In particular if the structure on the substrate with this subsegmentation 1130 is illuminated with radiation of a certainpolarization, one of the configurations may scatter more light thanother configurations. In addition, the higher diffraction orders ofconfigurations 1132, 1132′ have, seen in a top view, another directionthan the higher diffraction order of configurations 1133, 1133′. Thedifferent directions of the higher diffraction orders enable that in theoptical systems of the previously discussed metrology apparatuses thehigher diffraction orders of one of the two different configurations maybe blocked by a specific detection pupil. Embodiments of detectionpupils have been discussed in the context of FIG. 5 and/or FIG. 12 . Ingeneral this form of sub segmentation has areas where features arearranged in different directions to distinguish, in the image that isformed on e.g. sensor 440, certain areas of the structure from otherareas of the structure.

A third embodiment of sub segmentation 1140 is presented at the middleright side of FIG. 11 . In this structure the pitch between all featuresis equal but the width of the lines varies from thin to wide to thin,etc. Thereby there are first areas 1142, 1142′ that may be imaged as afeature on the sensor 440 and there are second areas 1143, 1143′ thatmay not be imaged as a feature on the sensor 440. In this form of subsegmentation, the width of the lines are modulated according to aspecific pattern or function.

It is to be noted that in the above examples 1120, 1130, 1140 of subsegmentation, the sub segmentation was performed in a single dimensionper portion or area of the target. At the bottom right side of FIG. 11it has been show that in another example, the sub segmentation may beperformed in two dimensions. E.g. the area that is indicated in thethird embodiment 1130 of sub segmentation with 1133′ the subsegmentation consists of relatively small horizontal lines. In anotherexample, the relatively small features of the sub segmentation may be arepetitive pattern of small features in the x- and y-dimension asindicated at 1133″. For example, the relatively small horizontal linesmay be sub divided in relatively small rectangular features that areplaced in a two dimensional grid structure. In all examples of subsegmentation 1120, 1130, 1140 each presented line may also be subdivided in smaller line segments.

Creating sub segmentations may be useful for having a higher correlationbetween process effects on the target 1100 on the substrate 450 and theprocess effects that influence the product/in-die structures on thesubstrate 450. In other words, the target is more representative forproduct/in-die structures. Furthermore, because of design rulerequirements there is a need to prevent too wide lines and/or to preventrelatively large empty areas on the substrate 450. As discussed earlier,the sub segmentation may also be used to influence the contrast betweenthe different portions/areas of the target. The sub segmentationinfluences the diffraction efficiency of the portions/areas of thetarget. Thereby the sub segmentation influences the contrast between thedifferent portions/areas of the target in the image that is formed one.g. the sensor 440.

In the sub segmentations 1120, 1130, 1140 a pitch, line width andrelative line orientation may be selected such that the scatterefficiency is relatively large. In other words, the pitch, line widthsand relative line orientations are selected to have the scatterefficiency above a predefined threshold. Thus, a relative large amountof the illumination radiation is scattered into the higher diffractionorders such that the amount of radiation that impinges on the sensor 440is relatively high. Furthermore, a pitch, line width and relative lineorientation may be selected such that an optical contrast of an imageobtained by sensor 440 is relatively large. The relative large opticalcontrast may be beneficial to extract a relative accurate overlay valuefrom the image obtained by sensor 440.

Sub segmentations 1120, 1130, 1140 are examples of sub segmentationsthat are based on different principles. Within a certain target 1100different types of sub segmentation may be combined. The skilled personis also capable of applying some of the presented principles in twodimensions by creating, e.g., in sub-segmentation 1120 also asub-segmentation in the y-dimension.

In the discussion of the examples of FIG. 6 and FIG. 11 , it is assumedthat the targets 600, 650, 660, 1100 have structures 612, 1112 in afirst layer of a manufactured 3d structure on a wafer and havestructures 622 in a second layer of the manufactured 3d structure on thewafer. However, it is not necessary that the structures are provided indifferent layers. They might also be located in a single layer of themanufactured 3d structure on the wafer, but they are not manufactured inthe same lithographic step. For example, with double patterning or aLitho-Etch-Litho-Etch process one can manufacture during eachlithographic step different structures in the same layer of the 3dstructure. The structures that are manufactured during these differentlithographic steps may have a displacement with respect to each other asthe result of alignment errors and, consequently, overlay errors may bepresent between the structures. Thus, if the targets 600, 650, 660, 1100are manufactured by creating structures 612, 1112 with a firstlithographical step and the structures 622 with a second lithographicalstep, one can use the targets 600, 1100 also to measure overlay errorsthat are the result of the use of two lithographic steps. The twolithographic steps may be used to manufacture structures in differentlayers or in the same layer. It is to be noted that this may also applyto the targets 1020, 1030 of FIG. 10 .

FIG. 15 schematically presents a further embodiment of a target 1500.Target 1500 has a similar structure as targets 600, 1100 and, in so farapplicable, discussed characteristics of targets 600, 1100 may alsoapply to target 1500. In FIG. 15 boxes with dashed lines are shown.These boxes are groups of structures. The dashed lines are not presenton a substrate that is printed but are only used to indicate whichstructures for a group. Target 1500 has four groups 1510, 1520, 1530,1540 of structures. Two groups 1510, 1540 of structures can be used tomeasure overlay errors in a first dimension and the two other groups1520, 1530 of structures can be used to measure overlay errors in asecond dimension. The principles of determining overlay errors from animage that is created with the previously discussed metrology apparatusare discussed, for example, in the context of FIG. 7 . Only for onespecific group 1510 of structures, substructures 1511, 1512, 1513 areindicated. In the other groups 1520, 1530, 1540 of structures, thesubstructure having a hatching equal to the hatching of substructures1511, 1512, 1513 have the same characteristics of the substructures1511, 1512, 1513 as discussed hereinafter.

Substructures 1511, 1512, 1513 are manufactured with differentlithographic steps. They may be present in the same layer of the 3dstructure that is being manufactured on the wafer. They may also bepresent in different layers of the 3d structure that is beingmanufactured on the wafer. It is also possible that two types ofsubstructures 1511, 1512, 1513 are in the same layer, while the thirdtype of substructure 1511, 1512, 1513 is in another layer. Assuming thatsubstructures 1511 are manufactured by a first lithographic step,assuming that substructures 1512 are manufactured by a secondlithographic step, and assuming that substructures 1513 are manufacturedby a third lithographic step, then target 1500 may be used to determineoverlay errors between the first lithographic step and the secondlithographic step, between the second lithographic step and the thirdlithographic step and also between the first lithographic step and thethird lithographic step. By using target 1500 one may measure multipleoverlay errors in a one measurement step, while, when target 600 isused, at least two targets and two measurement steps were necessary.Thus target 1500 provides speed benefits and also a reduction of spaceon the wafer that is used by metrology targets.

It is to be noted that the example of target 1500 provides three typesof substructures 1511, 1512, 1513. Of course it is also possible toprovide more than two or three types of substructures such that with asingle target overlay errors between more layers and/or morelithographical steps can be measured.

It is also to be noted that in cases where not directly lithography, butother wafer processing steps are used to manufacture separate structuresin a single layer, the different types of substructures 1511, 1512, 1513may be used to measure overlay errors that are induced by the specificwafer processing steps, or combinations of different processing steps,optionally in combination with lithographical steps.

Further embodiments are disclosed in the subsequent numbered clauses:

1. A metrology apparatus for determining a characteristic of interestrelating to at least one structure on a substrate, the metrologyapparatus comprising

-   -   a sensor for detecting characteristics of radiation impinging on        the sensor,    -   an optical system being configured to illuminate the at least        one structure with radiation received from a source and the        optical system being configured to receive radiation scattered        by the at least one structure and to transmit the received        radiation to the sensor.

2. A metrology apparatus according to clause 1, wherein the opticalsystem comprises an illumination path from the source to the structureon the substrate and a detection path from the structure on thesubstrate to the sensor, wherein, optionally, a portion of theillumination path overlaps with the detection path.

3. A metrology apparatus according to any one of the clauses 1 and 2,wherein

-   -   the sensor is arranged in an image plane of the optical system        or the sensor is arranged in a plane conjugate with the image        plane, and    -   the optical system is configured to image the at least one        structure on the sensor    -   and wherein, optionally, features of the structure can be        individually distinguished in an image being formed on the        sensor.

4. A metrology apparatus according to any one of the preceding clauses,wherein the optical system is configured to prevent a transmission ofradiation of the 0^(th) diffraction order of the scattered radiationtowards the sensor.

5. A metrology apparatus according to clause 4, wherein the opticalsystem comprises a blocking element for blocking the transmission of the0^(th) diffraction order of the scattered radiation towards the sensor.

6. A metrology apparatus according to clause 4, wherein the blockingelement is controllable in a first position where the blocking elementis operable to block the transmission of the 0^(th) diffraction order ofthe scattered radiation towards the sensor and the blocking element iscontrollable in a second position where the blocking element does notblock the transmission of the 0^(th) diffraction order of the scatteredradiation towards the sensor.

7. A metrology apparatus according to any one of the clauses 5 and 6,wherein the blocking element is a mirror that reflects the 0^(th)diffraction order into a direction that is not towards the sensor.

8. A metrology apparatus according to clause 7, wherein the mirrorreflects the order diffraction order to a further sensor and,optionally, an imaging lens is provided in between the mirror and thefurther sensor for creating a bright field image of the structure on thefurther sensor.

9. A metrology apparatus according to any one of the preceding clauses,wherein the characteristic of interest is an overlay value between afirst layer and a second layer on the substrate and the at least onestructure comprising features in the first layer and in the secondlayer.

10. A metrology apparatus according to clause 9, wherein at least oneof:

-   -   the at least one structure comprises a repetitive structure of        features within a first area in the first layer,    -   the at least one structure comprises a repetitive structure of        features within a second area in the second layer,    -   and, optionally, a pitch of the features in the first layer is        substantially identical to a pitch of the features in the second        layer.

11. A metrology apparatus according to clause 10, wherein at least oneof

-   -   in a top view of the substrate, the first area and the second        area are at least partially non-overlapping,    -   the first area and the second area are adjacent to each other,    -   in a top view of the substrate, the first area and the second        area do not overlap.

12. A metrology apparatus according to any one of the preceding clauses

-   -   further comprising a determining system    -   being configured to receive a signal from the sensor        representing an image being recorded on the sensor and    -   being configured to determine the overlay value on basis of a        displacement of the features in a first layer of the substrate        with respect to the features in a second layer of the substrate,        wherein the displacement is determined on basis of the image.

13. A metrology apparatus according to any one of the preceding clauseswherein the optical system comprises one or more lenses wherein at leasta subset of the one or more lenses is configured to focus the receivedradiation from the source in a spot on the structure and wherein atleast a further subset of the lenses is configured to transmit theradiation scattered by the at least one structure to the sensor.

14. A metrology apparatus according to clause 13 wherein the one or morelenses are configured to image the at least one structure on the sensor.

15. A metrology apparatus according to any one of the clauses 13 and 14,wherein the one or more lenses have a small aberration.

16. A metrology apparatus according to any one of the preceding clauses,wherein the optical system has a numerical aperture that is larger than0.5, or optionally larger than or optionally larger than 0.9.

17. A metrology apparatus according to any one of the preceding clauses,wherein a numerical aperture of the optical system is high enough tocapture at least one of a plus and a minus first diffraction order ofthe scattered radiation for transmission to the sensor.

18. A metrology apparatus according to any one of the preceding clausesfurther comprising the source.

19. A metrology apparatus according to clause 18, wherein the source isconfigured to generate radiation having one or more wavelengths in awavelength range from 200 nm to 2000 nm, or optionally, in a range from300 nm to 1500 nm, or optionally, in a range from 400 nm to 800 nm.

20. A metrology apparatus according to any one of the clauses 18 and 19,wherein the source is configured to generate, in use, a radiation havinga power larger than 50 Watt, or optionally, larger than 150 Watt, oroptionally, larger than 250 Watt, or optionally, larger than 1000 Watt.

21. A metrology apparatus according to any one of the preceding clauses,the sensor having a signal/noise level higher than 1, or optionally,higher than 10.

22. A metrology apparatus according any one of the preceding clauseswherein the sensor comprises an array of pixels for generating an imageof impinging radiation.

23. A metrology apparatus according to any one of the preceding clauseswherein the optical system comprises a first polarizer for polarizingthe radiation that illuminates the at least one structure, the firstpolarizer being provided in the illumination path.

24. A metrology apparatus according to any one of the preceding clauseswherein the optical system comprises a second polarizer for allowing thetransmission of scattered radiation to the sensor that has a particularpolarization, the second polarizer being provided in the detection path.

25. A metrology apparatus according to any one of the clauses 23 or 24,wherein at least one of the first polarizer and the second polarizer isa controllable polarizer being capable to allow the transmission ofradiation having a specific polarization as indicated in a polarizationcontrol signal.

26. A metrology apparatus according to any one of the preceding clauseswherein the optical system comprises a wavelength filter for allowingthe transmission of radiation within a wavelength range, the wavelengthfilter being provided in at least one of the illumination path and thedetection path of the optical system.

27. A metrology apparatus according to any one of the preceding clauseswherein the wavelength filter is a controllable wavelength filter beingcapable to allow the transmission of radiation in a selectablewavelength range in dependence of a wavelength control signal.

28. A metrology apparatus according to any one of the preceding clauseswherein the optical system and/or the source is configured to enable theillumination of the at least one structure with radiation having awavelength that results in capturing at least one higher diffractionorder of the scattered radiation by the optical system for transmissionto the sensor.

29. A metrology apparatus according to any one of the preceding clauseswherein the optical system and/or the source is configured to enable theillumination of the at least one structure with a plurality ofwavelengths.

30. A method of determining a characteristic of interest relating to atleast one structure on a substrate, the method comprising:

-   -   illuminating the structure with radiation via an illumination        path of an optical system,    -   receiving radiation being scattered by the structure on a sensor        via a detection path the optical system.

31. A method of determining a characteristic of interest relating to atleast one structure on a substrate, the characteristic of interest is anoverlay value between a first layer and a second layer on the substrateand the at least one structure comprising features in the first layerand in the second layer. the method comprising:

-   -   illuminating the structure with radiation via an optical system,    -   receiving radiation being scattered by the structure with the        optical system, the optical system comprising a sensor in an        image plane of the optical system or a plane conjugate with the        image plane,    -   imaging the at least one structure on the sensor with the        optical system while preventing a transmission of the 0^(th)        diffraction order of the scattered radiation towards the sensor,    -   recording an image with the sensor,    -   determining the overlay value on basis of a displacement of the        features in the first layer of the substrate with respect to the        features in the second layer of the substrate, the displacement        being determined on basis of the image.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

1. A computer program product stored on a non-transitory computerreadable medium and comprising a set of instructions that, whenexecuted, cause a processor to execute processes of: causing an opticalsystem to illuminate, using a radiation beam from a radiation source, atleast one structure on a substrate, the at least one structurecomprising first repetitive features at a first pitch in a first layerand second repetitive features at a second pitch in a second layer, thefirst repetitive features at least partially overlapping with the secondrepetitive features, and the first pitch being different from the secondpitch; causing the optical system to receive radiation scattered by theat least one structure and transmit a portion of the received scatteredradiation to a sensor, the sensor configured to be arranged in an imageplane of the optical system or in a plane conjugate with the image planefor detecting the received scattered radiation and configured to detecta characteristic of radiation impinging on the sensor; and determining acharacteristic of interest of the at least one structure on thesubstrate based on the radiation transmitted to the sensor.
 2. Thecomputer program product of claim 1, wherein the processor furtherexecutes a process of controlling the optical system to prevent or allowtransmission of radiation of the 0^(th) diffraction order of thescattered radiation towards the sensor.
 3. The computer program productof claim 2, wherein the process of controlling the optical system toprevent or allow transmission of radiation of the 0^(th) diffractionorder of the scattered radiation towards the sensor comprisescontrolling a position of an optical element that is configured toprevent or allow the transmission of the radiation of the 0^(th)diffraction order of the scattered radiation towards the sensor.
 4. Thecomputer program product of claim 3, wherein the processor causes theoptical element to move from a first position to a second position bymeans of a translation or a rotation of the optical element.
 5. Thecomputer program product of claim 2, wherein the optical system iscontrolled so that radiation of the 0^(th) diffraction order of thescattered radiation is reflected towards a second sensor.
 6. Thecomputer program product of claim 1, wherein the determining thecharacteristic of interest of the at least one structure on thesubstrate comprises determining an overlay value of the first repetitivefeatures and the second repetitive features.
 7. The computer programproduct of claim 6, wherein the determining an overlay value of thefirst repetitive features and the second repetitive features comprisesdetermining a displacement between the first repetitive features and thesecond repetitive features.
 8. The computer program product of claim 1,wherein the determining the characteristic of interest of the at leastone structure on the substrate comprises providing information about anexpected or ideal structure to be detected.
 9. The computer programproduct of claim 8, wherein the determining the characteristic ofinterest of the at least one structure on the substrate furthercomprises applying pattern recognition to an image at the sensor todetect a structure similar to the expected or ideal structure.
 10. Thecomputer program product of claim 1, wherein the determining thecharacteristic of interest of the at least one structure on thesubstrate comprises providing one or more parameters of the measurement.11. The computer program product of claim 10, wherein the one or moreparameters of the measurement includes at least one of a wavelength,polarization, intensity distribution, and an illumination angle ofillumination radiation.
 12. The computer program product of claim 10,wherein the one or more parameters of the measurement includes at leastone of a number of measured points and one or more locations of measuredpoints.
 13. The computer program product of claim 1, wherein thedetermining the characteristic of interest of the at least one structureon the substrate comprises providing one or more parameters of thestructure.
 14. The computer program product of claim 13, wherein the oneor more parameters of the structure includes at least one of a geometriccharacteristic, an orientation, a pitch, a size, and a segmentation of afeature in the structure.
 15. The computer program product of claim 13,wherein the one or more parameters of the structure includes at leastone of a length the first repetitive feature or the second repetitivefeature, a materials property of the structure, and an identification ofthe structure.
 16. The computer program product of claim 15, wherein:the one or more parameters of the structure includes a materialsproperty of at least a part of the structure, and the materials propertyincludes at least one of a refractive index, an extinction coefficient,and a material type of the at least a part of the structure.
 17. Thecomputer program product of claim 1, wherein the processor furtherexecutes a process of controlling actuators to move the at least onestructure with respect to a spot of the beam of radiation thatilluminates the at least one structure.
 18. The computer program productof claim 1, wherein the processor further executes a process ofcontrolling a wavelength filter to transmit radiation of a predeterminedwavelength or radiation having a wavelength within a predeterminedrange.
 19. The computer program product of claim 1, wherein theprocessor further executes a process of controlling a polarization of atleast one of the beam of radiation and the scattered radiation.
 20. Thecomputer program product of claim 1, wherein the processor furtherexecutes a process of controlling a wavelength of the beam of radiationemitted by the radiation source.